The demand for gasoline as a motor fuel is one of the major factors which dictates the design and mode of operation of a modern petroleum refinery. The gasoline product from a refinery is derived from several sources within the refinery including, for example, gasoline from the catalytic cracking unit, straight run gasoline, reformate and gasoline obtained as a low boiling by-product from various refinery operations, especially catalytic processes such as catalytic dewaxing. The octane number of the gasoline from these different sources varies according to the nature of the processing and the octane rating of the final gasoline pool will depend upon the octane ratings of the individual components in the pool as well as the proportions of these components. The increasing use of unleaded gasoline coupled with increasing engine efficiencies in road vehicles has led to a demand for increased gasoline pool octane which, in turn, makes it desirable to increase the octane values of the individual components of the pool. Although there are various ways of achieving this objective, some necessarily involve compromises which may render them less attractive in a commercial refinery operation. For example, the octane rating of FCC gasoline may be improved by operating the cracker at a higher temperature (conventionally measured at the top of the riser). Similarly, reformate octane may be increased by operating the reformer at higher severity. However, in both cases, a yield loss will ensue.
In the case of by-product gasoline from catalytic dewaxing processes it may be possible to improve octane during the start-up by increasing the temperature rapidly to a value higher than normal, as described in U.S. Pat. No. 4,446,007 (Smith). However, the use of higher temperatures in dewaxing processes will also tend to decrease the yield of dewaxed products. Alternative measures for increasing pool octane are therefore still desirable.
Another trend which is perceptible in the petroleum refining industry is towards the reduction of benzene and other lower boiling point aromatics in the gasoline or distillate pool. In the United States, the Environmental Protection Agency (EPA) is considering regulation of the gasoline content and similar measures are being considered in the European Community. Benzene is particularly prevalent in reformer gasoline, being a distinctive product of the reforming process, produced by the dehydrogenation of C.sub.6 cycloparaffins, the dehydrocyclization of straight chain paraffins of appropriate chain length (C.sub.6) and dealkylation of other aromatics. It is produced in particularly high concentrations in the continuous catalytic reforming process which is currently replacing the conventional cyclic reforming process in the industry. It would be possible to reduce the benzene content of the reformate by a simple fractionation process but because the boiling point of benzene is close to that of other desirable and unobjectionable components of the reformate, this too would lead to a considerable loss in yield.
Therefore, what is needed is a process for selectively removing certain aromatic components contained in a mixture of aromatic hydrocarbons found in a refinery liquid fuel pool so as to reduce the aromatic content of a gasoline pool for environmental reasons and reduce soot formation in a distillate pool.